dpif-netdev: Fix memory leak in dpif_netdev_flow_put()

miniflow_destroy() needs to be called after using miniflow_init().
Otherwise, if the miniflow mallocs data, then a memory leak may
occur.

Found by inspection.

Signed-off-by: Ryan Wilson <wryan@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
CodingStyle: Functions in .c files should usually not be marked "inline".

I thought this was in here already, but I guess not.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>